The past several years have seen a large increase in commercial spoken dialog systems. These systems typically utilize system-initiative dialog strategies. The system utterances are highly scripted for style and then recorded by voice talent. However several factors argue against the continued use of these simple techniques for producing the system side of the conversation. First, the quality of text-to-speech systems has improved to the point of being a viable alternative to prerecorded prompts. Second, there is a perceived need for spoken dialog systems to be more flexible and support user initiative. However, this factor also requires greater flexibility for system utterance generation. Finally, there are dialog systems that support complex planning currently under development, and these systems are likely to require more sophisticated system output than current technologies will be able to provide.